IPv4 - Internet Protocol packet (datagram packet).
  Address
  4 octet current protocol - 2^32.
    (IPv6) - 2^128 - however it must be backwards compatible. 
       Stateless

  Packet or datagram details. IPv4

  Minimum of 40 bytes and up to 65535 bytes.  
    Packet size independent of LAN and router hop protocols.
    * IPv6 attempts to discover maximum packet size across the whole connection
      and limits the initial packet size.

  Besides data being transmitted, some information it contains:
    Total Length of packet up to 65535. 
    * IPv6 requires a query of route and limits size to smallest datalink
      frame discovered.
    
    Packet ID - serial number combined with Host ID results in a unique id.
 
    Fragment Offset - used if datagram larger than frame payload of particular
      network. Packet fragmented and serialized.

    TTL - time to live, used to guard against a lost and wandering packet.
    * IPv6 uses hop count, doesn't bother with time-stamp.

    Protocol number - type of packet: ICMP, UDP, TCP, others
      http://en.wikipedia.org/wiki/List_of_IP_protocol_numbers

    Source and destination IPs.

  Error checking - checksum at the IP level, just check that the packet been 
    delivered in an uncorrupted form. * IPv6 assumes error correction done at 
    data-link and drops use of checksum.